Antennas, such as used in mobile satellite communications systems, have differing requirements depending upon the particular application for the antenna. For an asset tracking application, for example, the ideal antenna would have horizon-to-horizon hemispherical coverage, have excellent circular polarization characteristics, and have a bandwidth sufficiently large to cover transmit and receive bands, while being compact and low cost.
Patch antennas may be used for applications such as GPS where circular polarization provides optimum link performance. Such antennas, although much more compact, have the disadvantage of a narrow bandwidth and are easily detuned due to their mode of operation. A reduction in antenna size is highly desirable for mobile communication systems. However, designers of antennas for systems using circular polarization (CP) have very few options, because of symmetry requirements associated with CP.
A patch antenna includes a resonant conductive patch and a conductive ground plane, both strategically disposed in a dielectric substrate. Patch antennas are approximately λG/2 in length, where λG is the guided wavelength. The guided wavelength can be made smaller by increasing the dielectric constant of the substrate separating the patch from the ground plane. A linearly polarized patch can be visualized as two radiating edges, which radiate in-phase because of the 180 degree phase shift between them, as shown in FIG. 1.
Compact CP antennas, such as those commonly used in GPS receivers, are made electrically small by using very high dielectric constant substrates, such as ceramics, to the detriment of bandwidth.
Other size reduction techniques include λG/4 patches with short circuit loading on one edge. Further size reduction can also be achieved by using short circuited folded patches and other variants of that kind. While those antennas can perform well as linearly polarized antennas, the use of shorting pins violates the orthogonal symmetry required for a CP operation. Examples of patch configurations using short circuit loading are shown in FIG. 2.
A slotted patch, a variation often used to create multi-band antennas, however, is amenable to circularly polarized operation because of its orthogonal symmetry. Multiple resonant modes may be created by the addition of the slots, but the lowest order (lowest frequency) resonant mode occurs at a frequency lower than a solid conductor patch of equivalent size. Equivalently, for a given frequency of operation, the slotted patch would be smaller. That configuration is illustrated in FIG. 3.
The aforementioned can be explained by considering the basic components of the patch antenna model of FIG. 1. Radiating edges still exist and are separated and fed by a 180-degree resonator (resonant conductive patch). By introducing slots in the patch, the electrical path taken by the lowest order mode is longer than it would be for a patch without slots. Hence, the 180-degree resonator can be made physically smaller.
A logical extension that could be made by someone skilled in the art could be to cut more slots in the patch, thereby further reducing the physical size of the resonant conductive patch. That idea is limited as there is a point when no more slots can be added that are sufficiently sized to further reduce the physical size of the 180° resonator. Thus, an unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies for circularly polarized patch antennas.